Discovery of the Electron: Cathode Ray Tube Experiment

Summary

In this educational video, the discovery of the electron by J.J. Thomson in 1897 is explored, challenging the indivisible atom model proposed by John Dalton. Using a cathode ray tube, Thomson demonstrated that atoms contain smaller, negatively charged particles—electrons. His experiments with charged plates and a magnet led to the conclusion that electrons are a thousand times smaller than hydrogen atoms, leading to the 'plum pudding' model of the atom. This foundational work laid the groundwork for further atomic discoveries, including the nucleus.

Takeaways

• 🧪 JJ Thompson discovered the electron in 1897 using a cathode ray tube experiment, challenging the idea of atoms being indivisible.
• 🌌 In the late 1800s, John Dalton's atomic model proposed that all matter was made up of tiny, indivisible particles called atoms.
• 🔬 Thompson's experiment involved evacuating air from a glass tube and observing the behavior of cathode rays when subjected to electrical charges and a magnetic field.
• 🚀 The cathode rays were found to be negatively charged particles, which were later identified as electrons.
• 📉 Thompson concluded that electrons were a thousand times smaller than hydrogen atoms, indicating the presence of subatomic particles within atoms.
• 🧲 The deflection of cathode rays by a magnet confirmed their negative charge, as they moved in the direction expected for negatively charged particles.
• 🔩 Thompson's experiments showed that different metals produced the same cathode rays, suggesting that electrons are a fundamental component of all atoms.
• 🍇 Thompson proposed the 'plum pudding' model of the atom, where electrons were embedded in a positively charged 'dough' to maintain electrical neutrality.
• 🔄 The plum pudding model was later disproved by Rutherford's gold foil experiment, which revealed a dense, positively charged nucleus at the center of the atom.
• 🔍 Thompson's work laid the foundation for further discoveries about atomic structure and the nature of subatomic particles.
• 🔬 The script emphasizes the scientific method, illustrating how experiments and observations lead to new theories and models of atomic structure.

Q & A

Who discovered the electron and in what year?

J.J. Thomson discovered the electron in 1897.

What was the prevailing atomic model before J.J. Thomson's discovery?

Before Thomson's discovery, the prevailing atomic model was John Dalton's model, which proposed that all matter was made up of tiny indivisible particles called atoms.

What is a cathode ray tube and how was it used in Thomson's experiment?

A cathode ray tube is a sealed glass tube from which air has been evacuated, with two pieces of metal at one end connected to a power source. Thomson used it to observe rays shooting from one piece of metal to another, creating a glowing spot when hitting a special coating on the glass.

How did Thomson determine that the cathode rays were negatively charged?

Thomson determined that the cathode rays were negatively charged by placing two metal plates on either side of the cathode ray tube, charging them, and observing the rays bending towards the positively charged plate.

What additional method did Thomson use to confirm the charge of the cathode rays?

Thomson used a magnet to further confirm the charge of the cathode rays. When he placed a magnet around the cathode ray tube, the rays bent in a direction that would be expected for negatively charged particles in a magnetic field.

What conclusions did Thomson draw about the size of the particles in the cathode rays?

Thomson concluded that the particles in the cathode rays were a thousand times smaller than a hydrogen atom, indicating that they were very small compared to atoms.

How did the type of metal used in the cathode ray tube experiments affect the cathode rays?

Thomson found that regardless of the type of metal used in the cathode ray tube, all metals gave off the same cathode rays, and the size of the particles in the rays remained the same.

What model of the atom did J.J. Thomson propose based on his experiments?

J.J. Thomson proposed the 'plum pudding' model of the atom, in which the atom is like a muffin with electrons as the blueberries embedded in a positively charged 'dough' that balances out the negative charge of the electrons.

How did Thomson's discovery of the electron impact the understanding of atomic structure?

Thomson's discovery of the electron disproved the idea of atoms being indivisible, showing that atoms are made up of smaller subatomic particles, specifically electrons.

What experiment followed to further refine the understanding of the atomic structure after Thomson's model?

After Thomson's model, it was Rutherford's gold foil experiment that disproved the plum pudding model and led to the discovery of a very small, very dense, positively charged nucleus at the center of the atom.

Outlines

00:00

🔬 JJ Thompson's Discovery of the Electron

In this section, the video discusses the groundbreaking discovery of the electron by JJ Thompson in 1897. It begins by contrasting the modern understanding of atoms as made up of subatomic particles with the prevailing view in the late 1800s, which was based on John Dalton's atomic model proposing atoms as indivisible. Thompson's use of a cathode ray tube experiment is highlighted, where he observed rays that bent in the presence of electric and magnetic fields, indicating they were negatively charged. This led to the conclusion that atoms contain smaller particles, electrons, which challenged the notion of atoms being indivisible.

05:03

🌟 The Significance of Electrons in Atomic Structure

This paragraph delves into the implications of Thompson's findings on the atomic model. It explains how Thompson's experiments with different metals and the consistent production of cathode rays led him to deduce that all atoms contain electrons. The video clarifies misconceptions about the experiment's relevance to atomic structure, emphasizing that since all matter is composed of atoms, the source of electrons must be within atoms themselves. Thompson's conclusions prompted a revision of the atomic model, introducing the concept of a positively charged substance balancing the negative charge of electrons, which he visualized as a 'plum pudding' model, where electrons are embedded in a positively charged medium.

10:05

🍰 The Plum Pudding Model and Its Limitations

The final paragraph of the script addresses the 'plum pudding' model proposed by JJ Thompson, which suggested that atoms consist of negatively charged electrons scattered within a positively charged substance, akin to blueberries in a muffin. However, the video points out that this model was later disproved. It hints at the future discovery of the atomic nucleus by Rutherford, which revealed that the positive charge and most of the mass of an atom are concentrated in a small, dense center, rather than being spread out as Thompson's model suggested. The video encourages viewers interested in the evolution of atomic models to explore Rutherford's gold foil experiment for further insights.

Keywords

💡JJ Thompson

JJ Thompson, also known as Joseph John Thomson, was a British physicist renowned for his discovery of the electron. In the video, he is credited with challenging the prevailing atomic model of the time, which was based on the idea of atoms being indivisible. Thompson's experiments with cathode ray tubes led to the groundbreaking conclusion that atoms contained smaller, negatively charged particles, which we now know as electrons.

💡Cathode Ray Tube

A cathode ray tube is a sealed glass tube with two metal electrodes, used in early experiments to study the properties of cathode rays. In the script, Thompson uses this device to observe the behavior of rays emitted from the negatively charged electrode, which ultimately led to the discovery of the electron. The tube's vacuum allowed for the observation of these rays without interference from air particles.

💡Electron

The electron is a subatomic particle that carries a negative electric charge. It is one of the first fundamental particles to be discovered, as detailed in the video script. Thompson's experiments with cathode rays revealed the existence of electrons, showing that they are much smaller than atoms and are a component of the atomic structure.

💡Subatomic Particles

Subatomic particles are particles that are smaller than an atom. The video discusses the discovery of the electron as an example of such a particle. This term is central to understanding the shift from the idea of atoms being indivisible to the modern atomic model, which includes various subatomic particles like protons, neutrons, and electrons.

💡John Dalton

John Dalton was an English chemist and physicist who proposed the atomic theory in the early 1800s. His model suggested that atoms were the smallest indivisible units of matter. The video script contrasts Dalton's model with Thompson's findings, highlighting the evolution of scientific understanding of atomic structure.

💡Atomic Model

An atomic model is a theoretical construct that describes the nature of atoms and their constituents. The video script outlines the transition from Dalton's atomic model, which considered atoms indivisible, to Thompson's model, which introduced the concept of atoms containing smaller particles like electrons.

💡Indivisible

The term 'indivisible' in the context of the video refers to the historical belief that atoms were the smallest possible units of matter and could not be broken down into smaller components. Thompson's discovery of the electron challenged this notion, showing that atoms are composed of even smaller particles.

💡Electrical Charge

Electrical charge is a fundamental property of certain subatomic particles, like electrons, which can be either positive or negative. In the video, Thompson's experiments with cathode rays revealed that these rays were negatively charged, leading to the identification of electrons as negatively charged particles.

💡Magnet

A magnet is an object that produces a magnetic field and can attract or repel other magnetic materials. In the script, Thompson uses a magnet to deflect cathode rays, providing further evidence that they are composed of negatively charged particles, as magnets interact with moving charged particles in predictable ways.

💡Plum Pudding Model

The plum pudding model, also known as the Thomson model, was an early attempt to describe the structure of the atom. Thompson proposed that atoms consisted of a positively charged 'dough' with negatively charged electrons embedded within, similar to plums in a pudding. The video script explains how this model was later disproved by Rutherford's gold foil experiment.

💡Rutherford's Gold Foil Experiment

Ernest Rutherford's gold foil experiment is a famous experiment that led to the discovery of the atomic nucleus. The video script mentions this experiment as the next step in the understanding of atomic structure after Thompson's work. Rutherford's findings contradicted the plum pudding model, revealing that the positive charge of an atom is concentrated in a central nucleus.

Highlights

JJ Thompson discovered the electron in 1897 using a cathode ray tube experiment.

In the late 1800s, the atomic model was based on John Dalton's theory of indivisible atoms.

Thompson's discovery showed that atoms are not indivisible and are made up of smaller subatomic particles.

The cathode ray tube experiment involved a vacuum glass tube with two metal electrodes.

Electrons were observed to shoot from one metal electrode to the other, creating a glowing spot.

Thompson determined that the cathode ray was negatively charged by using charged metal plates.

A magnet was used to confirm the negative charge of the cathode ray due to its deflection.

Thompson concluded that the particles in cathode rays were a thousand times smaller than a hydrogen atom.

Different metals in the cathode tube produced the same cathode rays, indicating a universal presence of electrons.

Thompson proposed that atoms contain tiny negatively charged particles, later identified as electrons.

The discovery challenged the idea that atoms were the smallest units of matter.

Thompson's model suggested that atoms are electrically neutral due to a balance of positive and negative charges.

The 'plum pudding' model was proposed, with electrons embedded in a positively charged 'dough'.

Thompson's model was later disproved by Rutherford's gold foil experiment, revealing a dense, positively charged nucleus.

The plum pudding model was an important step in understanding atomic structure before the discovery of the nucleus.

Thompson's work laid the foundation for further exploration into subatomic particles and atomic structure.

Transcripts

in this video we're going to talk about

how JJ Thompson discovered the electron

in 1897 using the cathode ray tube

experiment so these days we think about

atoms kind of like this they're made up

of a variety of smaller subatomic

particles but in the late 1800s it was

all about John Dalton's model of the

atom in the early 1800s John Dalton had

proposed that all matter all stuff was

made up of tiny indivisible particles

called atoms at first not many people

believed him but throughout the course

of the eighteen hundreds more and more

scientists got on board with his idea

but it remained an open question whether

the atoms were really indivisible were

they like tiny little billiard balls you

know hard and solid or were they made of

smaller things no one really knew the

answer that but JJ Thompson was able to

answer part of that question when he

discovered the electron he discovered

that it was much smaller than atoms he

discovered that Adams had electrons and

so he was able to say for the first time

that atoms weren't totally solid and

indivisible but they were made up of

smaller subatomic particles let's look

at how he discovered the electron so

Thompson used what's called a cathode

ray tube it's a big tube of glass that's

kind of like a bottle you can imagine

it's sort of like this soda bottle it's

sealed all over and then you pump all

the air out of it

okay and it has these two pieces of

metal at this end so Thompson connected

these two pieces of metal to a power

source and here's what happened our ray

shot from this piece of metal across the

tube and created a glowing spot over

here when it hit a special coating on

the inside of the glass now Thompson

didn't know what was going on but

really was going on is that electrons

from this piece of metal

we're shooting out they were attracted

to this piece of metal which has an

opposite charge but they were moving so

fast they shot all the way across the

length of the tube anyway

Thompson sees this ray and he gets

curious as to what it's made of

and so he asks the question the stuff

that the cathode ray is made of does it

have a electrical charge and here's how

he attempts to answer that question he

takes two metal plates and puts them on

either side of the cathode ray tube and

then he turns on electricity which makes

the top plate positively charged and the

bottom plate negatively charged and

check out what happens to the cathode

ray when he turns on the power he see

that he sees it now instead of going

straight through the cathode ray bends

up it bends up towards the positively

charged plate so based on this he

reasons that the cathode ray must be

made of stuff that's negatively charged

since it's attracted to the positively

charged piece of metal and he knows that

opposite charges attract

but just because he wants to be very

careful he's a very careful guy he tries

to check this another way so he uses a

magnet if something has an electrical

charge and it's moving a magnet will

push it in particular directions so he

takes a cathode ray tube if you want to

think that this marker is kind of a

cathode ray tube and he surrounds it

with a magnet just like this we can kind

of show what goes on by slipping the

magnet under like this so anyway he puts

the magnet here and he sees now that the

cathode ray bends in the other direction

it now moves downward now if the cathode

ray is indeed

negatively-charged this is exactly how

he would expect it to move in the

presence of a magnet like this so this

is just further confirmation that

whatever makes up the cathode ray

probably is negatively charged so he

takes the the data that he gets from

these experiments and he's able to draw

a few important conclusions about

cathode rays and what makes up atoms

okay so based on the outcomes of the

experiments with the charge plates and

the magnet Thompson is able to conclude

that cathode rays must be made of stuff

that is negatively charged okay he

combines the information that he got

from the experiments that I just told

you about with some other data that he

collected and he's able to come to the

conclusion that the particles that make

up cathode rays are a thousand times

smaller than a hydrogen atom hydrogen

atoms are the smallest atoms so this

thing that's in the cathode rays must be

really really tiny compared to an atom

and finally Thompson swaps out the type

of metal that's in the cathode tube he

uses a variety of different types of

metal but he finds that all of the

different metals he use he uses give off

the same cathode rays and regardless of

what metal he uses via the size of the

things that are in the cathode rays is

exactly the same so based on these three

conclusions he's able to come up with

the idea that atoms have tiny negatively

charged particles inside them and these

things that we're talking about here are

electrons now if you're like me when I

first learn this you might be confused

how these conclusions definitely tell

him that there are electrons inside

atoms I remember when I first learned

this stuff I was thinking like wait

doesn't this just show

electricity or cathode rays have

negatively charged stuff how does this

tell us anything about the atoms okay

here's why at the time that Thompson

does his experiment everyone for the

most part agrees that all things are

made up of atoms that atoms are the

smallest things that make everything up

there like the smallest things in the

universe

so if Thompson is able to show that

there are even tinier things well where

are those tiny things coming from they

have to be coming from atoms because

everything's made of atoms right that's

the only place they can be coming from

and even if electricity even if you want

to say that you know oh well he's just

showing that electricity is made of

electrons

well then still where are the electrons

coming from that make the electricity

the electrons have to be coming from

other atoms and since he can move and

remove these pieces of metal and swap

other metals in and he still gets

electrons it again shows that all

different types of atoms are able to

release or give off electrons so they

must be in there to begin with anyway

these conclusions that Thompson comes to

have dramatic effects on how scientists

think about what the atom actually looks

like now at the beginning of the video

we said that in Dalton's atomic model

atoms are indivisible that they're not

made of anything smaller

but when Thompson discovered that atoms

have tiny negatively charged electrons

inside them he essentially disproved

this model at least the part about the

indivisible atoms so here is the model

of the atom that Thompson comes up with

he realizes first of all that electrons

are negative we already talked about

that but then he notices that on the

whole atoms in general are usually

electrically neutral

so even though they have these

negatively charged electrons inside them

the atoms as a whole don't have a charge

so Thompson realizes that in order for

the atoms not to have a charge there

must be some positive charge in the atom

that can balance out the negative charge

of the electrons and then the positive

and the negative balance out and the

atom that as a whole doesn't have a

charge so he comes up with a model that

he calls the plum pudding at it now plum

pudding is kind of a British thing so if

you're American it might be better for

you to think about this as like the

blueberry muffin model and here's how it

works

Thompson imagines that the atom is like

a blueberry muffin and that electrons

are stuck in this atom the way

blueberries are stuck in a blueberry

muffin so you have these little

electrons that are stuck in the atom but

then they're surrounded sort of by a

dough and the dough like in a blueberry

muffin

um keeps these electrons or blueberries

held in place but the dough itself has a

positive charge and it's that positively

charged dough that is what balances out

the negative charge of the electrons and

so together they don't have any total

net charge so the atom is electrically

neutral so that's how JJ Thompson's

discovery of the electrons leads to this

plum pudding model yeah but now JJ

Thompson didn't have it quite right

because it turns out that the positive

charge in the atom isn't he isn't sort

of spread out in this dough or pudding

but instead it's concentrated right in

the center of the atom where the nucleus

is so that's gonna be the next step in

discovering what the atom really looks

like if you're interested in how

scientists disproved JJ Thompson's plum

pudding model check out Rutherford's

gold foil experiment where he discovers

that atoms have a very small very dense

positively charged

nucleus